The Star that Changed the Universe

Though the universe is filled with billions upon billions of stars, the
discovery of a single variable star in 1923 altered the course of modern
astronomy. And, at least one famous astronomer of the time lamented that
the discovery had shattered his world view.

The star goes by the inauspicious name of Hubble variable number one,
or V1, and resides in the outer regions of the neighboring Andromeda
galaxy, or M31. But in the early 1900s, most astronomers considered the
Milky Way a single "island universe" of stars, with nothing observable
beyond its boundaries. Andromeda was cataloged as just one of many
faint, fuzzy patches of light astronomers called "spiral nebulae."

Milky Way Galaxy with "Spiral Nebula" Andromeda in lower left corner of image.

Were these spiral nebulae part of the Milky Way or were they independent
island universes lying outside our galaxy? Astronomers didn't know for
sure, until Edwin Hubble found a star in Andromeda that brightened and
faded in a predictable pattern, like a lighthouse beacon, and identified it
as V1, a Cepheid variable. This special type of star had already been
proven to be a reliable distance marker within our galaxy.

The star helped Hubble show that Andromeda was beyond our galaxy and
settled the debate over the status of the spiral nebulae. The universe
became a much bigger place after Hubble's discovery, much to the
dismay of astronomer Harlow Shapley, who believed the fuzzy nebulae
were part of our Milky Way.

Nearly 90 years later, V1 is in the spotlight again. Astronomers pointed
Edwin Hubble's namesake, NASA's Hubble Space Telescope, at the star once
again, in a symbolic tribute to the legendary astronomer's milestone
observation.

Astronomers with the Space Telescope Science Institute's Hubble Heritage
Project partnered with the American Association of Variable Star Observers
(AAVSO) to study the star. AAVSO observers followed V1 for six months,
producing a plot, or light curve, of the rhythmic rise and fall of the
star's light. Based on this light curve, the Hubble Heritage team scheduled
telescope time to capture images of the star.

"V1 is the most important star in the history of cosmology," says
astronomer Dave Soderblom of the Space Telescope Science Institute
(STScI) in Baltimore, Md., who proposed the V1 observations.

Edwin Hubble c. 1951 (Courtesy: Carnegie Observatories)

Andromeda Galaxy M31(Courtesy: T. Rector)

"It's a landmark discovery that proved the universe is bigger and chock
full of galaxies. I thought it would be nice for the Hubble telescope to
look at this special star discovered by Hubble, the man."

But Hubble Heritage team member Max Mutchler of the STScI says that
this observation is more than just a ceremonial nod to a famous
astronomer.

"This observation is a reminder that Cepheids are still relevant today," he
explains. "Astronomers are using them to measure distances to galaxies
much farther away than Andromeda. They are the first rung on the
cosmic distance ladder."

The observations were still tricky, though. "The star's brightness has a
gradual decline followed by a sharp spike upward, so if you're off by a
day or two, you could miss it," Mutchler explains.

Light curve of M31-V1 (Courtesy: AAVSO and STScI)

Using the Wide Field Camera 3, the Heritage team made four observations in
December 2010 and January 2011.
"The Hubble telescope sees many more and much fainter stars in the field
than Edwin Hubble saw, and many of them are some type of variable star,"
Mutchler says. "Their blinking makes the galaxy seem alive. The stars
look like grains of sand, and many of them have never been seen before."

For Soderblom, the Hubble observations culminated more than 25 years
of promoting the star. Shortly after Soderblom arrived at the Institute in
1984, he thought it would be fitting to place a memento of Edwin
Hubble's aboard the space shuttle Discovery, which would carry the
Hubble Space Telescope into space.

"At first, I thought the obvious artifact would be his pipe, but
[cosmologist] Allan Sandage [Edwin Hubble's protege] suggested another
idea: the photographic glass plate of V1 that Hubble made in 1923,"
Soderblom recalls.

He had made 15 film copies of the original 4-inch-by-5-inch glass plate. Ten
of them flew onboard space shuttle Discovery in 1990 on the Hubble
deployment mission. Fittingly, two of the remaining five film copies were
part of space shuttle Atlantis's cargo in 2009 for NASA's fifth servicing
mission to Hubble. One of those copies was carried aboard by astronaut
and astronomer John Grunsfeld, now the STScI's deputy director.

Edwin Hubble with pipe
(Courtesy: The Huntington Library)

Hubble Space Telescope
(Courtesy: NASA)

Telltale Star Expands the Known Universe

Prior to the discovery of V1 many astronomers thought spiral nebulae,
such as Andromeda, were part of our Milky Way galaxy. Others weren't so
sure. In fact, astronomers Shapley and Heber Curtis held a public debate
in 1920 over the nature of these nebulae. During the debate, Shapley
championed his measurement of 300,000 light-years for the size of the
Milky Way. Though Shapley overestimated its size, he was correct in
asserting that the Milky Way was much larger than the commonly
accepted dimensions. He also argued that spiral nebulae were much
smaller than the giant Milky Way and therefore must be part of our
galaxy. But Curtis disagreed. He thought the Milky Way was smaller
than Shapley claimed, leaving room for other island universes
beyond our galaxy.

To settle the debate, astronomers had to establish reliable
distances to the spiral nebulae. So they searched for stars in the
nebulae whose intrinsic brightness they thought they understood.
Knowing a star's true brightness allowed astronomers to calculate
how far away it was from Earth. But some of the stars they selected
were not dependable milepost markers.

For example, Andromeda, the largest of the spiral nebulae, presented
ambiguous clues to its distance. Astronomers had observed different
types of exploding stars in the nebula. But they didn't fully
understand the underlying stellar processes, so they had difficulty
using those stars to calculate how far they were from Earth. Distance
estimates to Andromeda, therefore, varied from nearby to far away.
Which distance was correct? Edwin Hubble was determined to find out.

The astronomer spent several months in 1923 scanning Andromeda with the
100-inch Hooker telescope, the most powerful telescope of that era, at
Mount Wilson Observatory in California. Even with the sharp-eyed
telescope, Andromeda was a monstrous target, about 5 feet long at the
telescope's focal plane. He therefore took many exposures covering
dozens of photographic glass plates to capture the whole nebula.

He concentrated on three regions. One of them was deep inside a spiral
arm. On the night of Oct. 5, 1923, Hubble began an observing run that
lasted until the early hours of Oct. 6. Under poor viewing conditions,
the astronomer made a 45-minute exposure that yielded three suspected
novae, a class of exploding star. He wrote the letter "N," for nova,
next to each of the three objects.

Later, however, Hubble made a startling discovery when he compared the
Oct. 5-6 plate with previous exposures of the novae. One of the so-called
novae dimmed and brightened over a much shorter time period than seen
in a typical nova.

Hubble obtained enough observations of V1 to plot its light curve,
determining a period of 31.4 days, indicating the object was a Cepheid
variable. The period yielded the star's intrinsic brightness, which Hubble
then used to calculate its distance. The star turned out to be 1 million
light-years from Earth, more than three times Shapley's calculated
diameter of the Milky Way.

Taking out his marking pen, Hubble crossed out the "N" next to the
newfound Cepheid variable and wrote "VAR," for variable, followed by an
exclamation point.

For several months the astronomer continued gazing at Andromeda,
finding another Cepheid variable and several more novae. Then Hubble
sent a letter along with a light curve of V1 to Shapley telling him of his
discovery. After reading the letter, Shapley was convinced the evidence
was genuine. He reportedly told a colleague, "Here is the letter that
destroyed my universe."

By the end of 1924 Hubble had found 36 variable stars in Andromeda, 12
of which were Cepheids. Using all the Cepheids, he obtained a distance of
900,000 light-years. Improved measurements now place Andromeda at 2 million
light-years away.

"Hubble eliminated any doubt that Andromeda was extragalactic," says
Owen Gingerich, professor emeritus of Astronomy and of the History of
Science at Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass. "Basically, astronomers didn't know the distance to novae, so they
had to make a rough estimate as to where they were and therefore what
their absolute luminosity was. But that is on very treacherous ground.
When you get a Cepheid that's been reasonably calculated, the period will
tell you where it sits on the luminosity curve, and from that you can
calculate a distance."

Shapley and astronomer Henry Norris Russell urged Hubble to write a
paper for a joint meeting of the American Astronomical Society and
American Association for the Advancement of Science at the end of
December 1924. Hubble's paper, entitled "Extragalactic Nature of Spiral
Nebulae," was delivered in absentia and shared the prize for the best
paper. A short article about the award appeared in the Feb. 10, 1925,
issue of The New York Times. Gingerich says Hubble's discovery was not
big news at the meeting because the astronomer had informed the
leading astronomers of his result months earlier.

Edwin Hubble's observations of V1 became the critical first step in
uncovering a larger, grander universe. He went on to find many galaxies
beyond the Milky Way. Those galaxies, in turn, allowed him to determine
that the universe is expanding.

Could Hubble ever have imagined that nearly 100 years later,
technological advances would allow amateur astronomers to perform
similar observations of V1 with small telescopes in their backyards? Or,
could Hubble ever have dreamed that a space-based telescope that bears
his name would continue his quest to precisely measure the universe's
expansion rate?